Support legacy _links LFS batch response.
Fixes#31512.
This is backwards-compatible change to the LFS client so that, upon
mirroring from an upstream which has a batch api, it can download
objects whether the responses contain the `_links` field or its
successor the `actions` field. When Gitea must fallback to the legacy
`_links` field a logline is emitted at INFO level which looks like this:
```
...s/lfs/http_client.go:188:performOperation() [I] <LFSPointer ee95d0a27ccdfc7c12516d4f80dcf144a5eaf10d0461d282a7206390635cdbee:160> is using a deprecated batch schema response!
```
I've only run `test-backend` with this code, but added a new test to
cover this case. Additionally I have a fork with this change deployed
which I've confirmed syncs LFS from Gitea<-Artifactory (which has legacy
`_links`) as well as from Gitea<-Gitea (which has the modern `actions`).
Signed-off-by: Royce Remer <royceremer@gmail.com>
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